They don't call 'em cancer sticks for nothing: Cigarettes kill more than 5 million people each year, including nearly 175,000 American women. But what if there were a safer—harmless, even—way to light up? Something that looks and feels like smoking but isn't, exactly; something that could even help you quit the real thing?

Such is the promise of electronic cigarettes. First popularized in 2003, e-cigs are now a $1.5 billion business, with 300 U.S. brands and 4 million current users, half of them women. They come in an array of sizes and colors; some are flavored (strawberry! mint!) and boast celebrity endorsements; all vow that they don't stain teeth or make you reek like eau de ashtray.

Indeed, "vaping," the term for e-smoking, has gained a reputation for healthfulness: Nearly 80 percent of e-cig users think the sticks do less damage than regular butts, per the American Journal of Preventive Medicine. The same study found that young women are the most likely to try them. What's more, while the majority of e-cig users were already smokers, there's now a subset of nonsmokers looking to experiment.

Absent in all this is the crucial question: How safe are e-cigs? Are vapers merely trading one evil for another—or are these new tools a bright light in an otherwise killer industry? The World Health Organization (WHO), for one, isn't sure, and neither are the FDA or the Centers for Disease Control and Prevention (CDC). All have expressed concerns about e-cig safety. At press time, the American Cancer Society was calling for restrictions, and the FDA was deciding how and when to regulate e-cigs—for apparently good reasons. (In fact, Los Angeles recently banned the use of e-cigarettes in public spaces. Similar legislation has passed in New York, Boston, and Chicago.)

What's Inside?
The anatomy of an e-cig is ostensibly simple: As you suck on the mouthpiece, a tiny battery ignites to heat a liquid nicotine solution until it becomes vapor, which you then inhale. Depending on the type and brand, an e-cigarette can last anywhere from 250 to 400-plus puffs (one real cig lasts for about 10 drags). Some are ready-to-use disposables ($4 and up per cigarette), others are rechargeable ($30 and up), and still others take refillable cartridges (up to $325).

Missing from e-cigs are tobacco and the more than 4,000 chemicals, many of them carcinogenic, that you'd get with a typical smoke (think ammonia, arsenic, tar). But the most important ingredient in both versions is nicotine, which can have some nasty long-term effects, says cardiologist Suzanne Steinbaum, D.O., author of Dr. Suzanne Steinbaum's Heart Book. The addictive stimulant constricts blood vessels, increases blood pressure, and restricts blood flow to your heart. It may cause airway problems and has been linked to disturbed sleep rhythms, depression, diabetes, even some cancers. It can also make you dizzy and irritable.

Perhaps most concerning, though, is that no one can know for certain how much is in any one e-cig, says Alex Prokhorov, M.D., Ph.D., director of the Tobacco Outreach Education Program at the University of Texas MD Anderson Cancer Center. Because it's unregulated, an e-cig cartridge could contain anywhere from one to 100 milligrams of nicotine (there's about one milligram in a traditional cig). And sucking in too much nicotine can be fatal. ("I've seen people OD on nicotine," says Steinbaum. "It's frightening.")

Researchers remain divided over e-cigarettes' other ingredients, usually a mix of water, flavoring, and propylene glycol. The latter is the chemical that creates the vapor and is "generally recognized as safe" by the CDC when used as a preservative in foods like salad dressing and ice cream. It's also, however, an ingredient in antifreeze and condoms—and the WHO says it's an irritant when inhaled. (Whether that leads to lung damage is still up for debate.)

And then there's the possibility of sketchy unnamed chemicals lurking in e-cigs. A series of 2009 FDA lab tests found that some e-cigs contain carcinogens such as formaldehyde. "The concentrations appear to be very low when compared with cigarettes," says Jonathan Foulds, Ph.D., an expert on tobacco addiction at Penn State University. "But we don't know the precise long-term health risks of decades of e-cig use." In other words: uncharted territory.

A Troubling Gateway
Independent of ingredient concerns, e-cig firms are catching flak for marketing to nonsmokers, especially teenagers. (It's the tobacco in cigarettes that's regulated, so currently there's no federal 18-and-over law for e-cigs.) This hook-'em-while-they're-young approach can add up to big bucks for manufacturers, many of whom are Big Tobacco companies. And since nicotine can be as addictive as cocaine, doctors worry e-cigs will act as "gateway" devices to actual smokes, says Jon Ebbert, M.D., of the Nicotine Dependence Center at the Mayo Clinic. In other words, as people get accustomed to—and hooked on—e-smoking, they might graduate to experimenting with more powerful real cigs.

Researchers estimate that to date, more than 1 million nonsmokers have tried vaping. The majority of users, however, were already smokers, including those desperate to kick the habit. E-cigarettes are not FDA-approved as quitting tools, though that hasn't discouraged some makers from dropping hints. One company sponsored its own study that found 70 percent of people who switched to e-cigs quit tobacco after 90 days. A study in The Lancet offers a more sobering perspective: After six months, only 7 percent of e-cig users quit cancer sticks.

For now, says Foulds, e-cigarettes aren't exactly awesome, but "if you're a smoker and there's an alternative that's equally addictive but much less likely to kill you, that's still somewhat of a win." There's that 7 percent chance they could help you quit entirely, and you'll be sparing yourself thousands of toxins. But if you aren't already a smoker, don't pick up an e-cig for fun. Unlike some vices (like, say, coffee or wine), these smokes come with zero health perks.

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